The control of marine fouling on ships and marine structures has been a problem for thousands of years. In the 20th century this problem has been addresed primarily by the use of coatings containing chemicals toxic to marine organisms. Conventional coatings used for fouling control are based on a film-forming resin and toxin additives such as cuprous oxide or triorganotin compounds which are slowly leached out of the coating by sea-water. Frequently the paint composition contains a slightly water-soluble resinous material such as gum rosin which assists the leaching process.
An example of such a "soluble matrix" paint formulation is given in Table 1.
TABLE 1 ______________________________________ Soluble Matrix Copper Oxide Antifouling Paint U.S. Navy 121/63 Formula Ingredient Pounds Gallons ______________________________________ Cuprous oxide 1440 50.0 Rosin 215 24.1 Vinyl resin (VYHH).sup.(a) 55 4.7 Tricresyl phosphate 50 11.7 Xylene 115 16.1 MIBK 165 24.7 Antisettling agent 7 1.0 ______________________________________ .sup.(a) Union Carbide
Such paint system, however, fail to provide a constant toxicant release, and moreover, do not erode in service. This is due to the selective extraction of the water-soluble component and consequent leaching of toxicant (cuprous oxide) from the interior of the paint film. A matrix of the insoluble vinyl resin component remains behind after the water-soluble component of the film (gum rosin) is leached away. Moreover, the spent paint film no longer controls fouling even though it might contain up to 30-40% of the initial level of cuprous oxide because water penetration required for leaching the copper to the surface is limited through the matrix of residual vinyl resin. Spent antifouling systems of this type do not provide a suitable base for repainting since they posses poor mechanical properties due to the voids in the film which result in poor adhesion of the new paint film.
Prior art attempts to incorporate toxicants into water soluble polymers and to use these as antifouling paints have also failed to produce the desired results. Such paints swell in seawater and cannot be expected to provide good mechanical properties and uniform control of fouling since the whole paint film is weakened on prolonged water immersion. Even such paint compositions as described in British Patent Specification No. 1,584,943 do not provide optimum control of fouling because the paint binder consists of a physical mixture of water insoluble and synthetic water-soluble polymeric binders wherein the synthetic water-soluble polymeric binder is substituted for the natural gum rosin of the previously described paint system. In the paint systems of British Patent Specification No. 1,584,943, the water-soluble polymeric component can be selectively extracted from the binder system by seawater leading to the same problems encountered with traditional vinyl/rosin systems. Moreover, on prolonged immersion in water, some portion of the water-soluble resin component can cause the film to absorb water and swell throughout its thickness yielding a film with poor mechanical properties.
In recent years, so-called self-polishing antifouling coatings have become increasingly popular. These coatings are based on copolymers of tributyltin methacrylate and methyl methacrylate or terpolymers of tributyltin methacrylate, methyl methacrylate and 2-ethylhexyl acrylate or butyl methacrylate. The organotin copolymer acts as the paint binder. All such paints also contain a toxicant additive such as cuprous oxide or a triorganotin compound. In addition the usual paint additives such as pigments, thixotropic agents, etc. may also be present. In normally alkaline seawater, the polymeric organotin binder is gradually hydrolyzed liberating bis(tributyltin)oxide which is an active antifoulant and also allowing the release of cuprous oxide or other physically-bound toxicants. The hydrolyzed polymer which is also formed is water-soluble or water-swellable and is easily eroded off the surface by moving sea-water, exposing a fresh surface of paint. The major advantage of these systems is that, unlike leaching paints, toxicant release is linear with time and all of the toxicant present is utilized over the lifetime of the paint. Furthermore, there is no need to remove the residues of an old self-polishing paint system prior to repainting, since the composition of the residue is essentially the same as it was when originally applied unlike conventional antifouling paints which leave a weak, leached-out matrix of binder on the ships' hull at the end of their lifetime. An additional advantage claimed for such systems is a reduction in hull surface roughness with time as a consequence of water-planing or erosion of the paint film. This roughness reduction translates to fuel savings for the ship operator.
Such erodible, antifouling coatings based on organotin copolymers and the mechanism by which they function are described in Journal of Coatings Technology, Vol. 53, Number 678, pages 46-62. The organotin copolymer serves two purposes in these systems. First it serves as a reservoir of tributyltin oxide which is gradually liberated over a period of time. Secondly it confers upon the polymer the unique property of sensitivity to hydrolysis by alkaline seawater. After hydrolysis, the polymer becomes seawater soluble or erodible. The erosion of such paint films is manifested by a gradual decrease of film thickness with time as the paint is exposed to moving seawater. In the laboratory, this erosion can be measured qualitatively by stripes of paint placed on a spinning disc immersed in seawater as described in U.S. Pat. No. 4,021,392 (Milne and Hails). Quantitative determination of erosion can be made by exposing panels coated with the test paint on a rotating drum immersed in seawater as described in Journal of Oil & Colour Chemists Association, Vol. 56, 1973, pages 388-395 and measuring the decrease of film thickness with time using a commercial electronic film thickness gauge.
Upon carrying out such erosion rate measurements on organotin acrylate or methacrylate copolymer films alone without any pigments or other additives, surprisingly it has been found that the erosion rate is a function of the reacted tributyltin methacrylate content of the copolymer. Moreover, there is an unexpected non-linear relationship between the polymer bound triorganotin content and the erosion rate.
Sea-going vessels usually have between 2 and 4 coats of antifouling paint, each coat of 100 microns film thickness, applied to the hull. This coating, of 200 to 400 microns total film thickness, is expected to last for two or three years which is the normal time between drydockings.
A simple calculation shows that to achieve the necessary lifetime, the erosion rate of such paints must fall in the range of 5 to 15 microns per month. An organotin copolymer which erodes at 12 microns per month must contain about 29 mole percent tributyltin methacrylate in the copolymer or about 46% by weight tributyltin groups. It has also been recognized by Milne and Hails in U.S. Pat. No. 4,021,392 that the organotin polymer must contain about 50% or more by weight of tributyltin salt units in order to generate the water-soluble polymer at a sufficient rate. A polymer with a surface dissolution rate of 12 microns per month and a specific gravity of 1.23, the normal range for such organotin polymers, release 49 micrograms of polymer per day for each square centimeter of surface area. Concommitantly, 23 micrograms of tributyltin are released per square centimeter per day which is equivalent to 9.4 micrograms of tin released from each square centimeter of ships' hull per day. Chromcy and Uhacz in Journal of the the Oil & Colour Chemists Associations, Volume 61, pages 39 to 42 (1978) have estimated that to control marine growth in the Baltic Sea, a release rate of between 0.4 and 0.7 micrograms of tributyltin per square centimeter per day is required. Even if it is assumed that in tropical waters this requirement must be increased five-fold, it is clear that polymeric organotin coatings release more organotin than is necessary to provide control of marine fouling. This is a consequence of the necessity to have a sufficient content or organotin in the polymer to allow the erosion mechanism by which these coatings function to proceed. This results in an unnecessarily large influx of toxicant into the environment and an unnecessarily high cost due to excess tin in the polymer. However it has been shown above that below this relatively high tin content, the polymers do not erode and the resulting antifouling coatings are not efficacious in controlling marine growth.
This can be demonstrated by preparing the same model paints using polymers of tributyltin methacrylate (TBTM) and methyl methacrylate with varying TBTM contents as the polymeric binder and evaluating their performance in static tests in Biscayne Bay, Fla. according to the following procedure.